Clojure macro example - dopar

From:
andrew cooke <andrew@...>

Date:
Sun, 10 Jun 2012 14:28:20 -0400

I just wrote my first "worthwhile" macro in Clojure (by which I mean, does
something non-trivial I expected the language to do for me).
I have some code I want to run multiple times. The only things that changes
each time is an integer, but each run takes 10 hours. So I had a loop that
took 10 hours per iteration. If you know Clojure then you'll know that my
code looked like this:
(doseq [i (range 4)]
(do-stuff-with i))
which is basically a "for loop" that changes "i".
To save time I wanted to run those things in parallel. So I wanted something
like Python's multiprocessing - a simple way to make "do-stuff-with i" run on
multiple cores.
Solving this went through three phases:
- Looking for an obvious existing solution. Couldn't find one.
- Trying to write an implementation using basic concepts from Java like
semaphores or ThreadPoolExecutor. This was complicated and "felt wrong".
- Working out how to do it the "Clojure way". Understanding that I needed
agents, and then writing a macro to manage the agents.
At first I was confused by agents - I thought they would be something like
processes. But really they are just values that a process can access. So in
my code above. So my original idea to put "do-stuff-with" in an agent was
wrong - it is the "i" that "goes inside" the agent.
So the basic way to solve this problem in Clojure is to:
- Create an agent for each "i"
- Use "send" to send the function "do-stuff-with" to each agent.
- Wait for all the agents to finish.
Behind the scenes, Clojure is careful to only let a few agents run at a time
(this is what "send" takes care of).
So a solution would look something like:
(apply await
(for [i (range 4)]
(let [a (agent i)]
(send a do-stuff-with)
a)))
which:
- Generates an agent for each i
- Sends the work to the agent (which is queued to run in a thread from a
thread pool)
- Creates a list of agents (the result of the "for")
- Waits for all the agents to finish.
OK, so where does the macro come into this? Well, this code:
(defmacro dopar [seq-expr & body]
(assert (= 2 (count seq-expr)) "single pair of forms in sequence
expression")
(let [[k v] seq-expr]
`(apply await
(for [k# ~v]
(let [a# (agent k#)]
(send a# (fn [~k] ~@body))
a#)))))
lets me write the (apply....) above as:
(dopar [i (range 4)]
(do-stuff-with i))
which is, I think, pretty awesome.
Andrew